1. Field of the Invention
This invention relates to the field of optimizing the performance of signal channels, and, more specifically, to optimizing the performance of integrated circuit disk drive read channels.
2. Background Art
Computer systems employ data storage devices, for example, disk drives, to store data for use by the computer system. A typical data storage device includes storage media, in which data is stored, a read head, and a mechanism, such as a motor, for imparting relative motion between the storage media and the read head. The relative motion allows access to various portions of the storage media, and, in the case of certain types of media, such as magnetic media, allows for the production of signals representative of the data stored in the storage media.
In general, disk memories are characterized by the use of one or more magnetic media disks mounted on a spindle assembly and rotated at a high rate of speed. Each disk typically has two surfaces of magnetic media. In a typical rotating medium as a storage system, data is stored on magnetic or magneto-optical disks in a series of concentric "tracks," with each track being an addressable area of the memory array. A read-write head is provided for each surface of each disk in the disk storage system. These tracks are accessed by a read-write head that detects variations in the magnetic orientation of the disk surface.
To provide retrieval of stored data from a storage medium, the fixed representation of the stored data in the storage medium must be converted into a signal that may be processed to yield data in a form usable with a system such as a computer system. A read channel circuit is used to convert signals from the storage media, for example a hard disk, to usable read data. A read channel circuit typically includes a pulse detector, a filter, servo circuits, a data synchronizer, a window shift circuit, a write precompensation circuit, an encoder/decoder (ENDEC), and a control circuit. The pulse detector detects and qualifies encoded read signals derived from the storage device. The filter further processes the encoded read signals to ensure frequency range and phase relationships of the encoded read signals are appropriate to allow read data to be recovered from the encoded read signals. The servo circuits capture servo information derived from the storage device which is used to assure that data to be read from the storage device has been accurately located.
In the read mode, the data synchronizer performs sync field search and data synchronization. The data synchronizer uses a phase locked loop (PLL) to provide data synchronization and to develop a decode window. The window shift circuit shifts the phase of the voltage controlled oscillator (VCO) of the PLL to effectively shift the relative position of the read data pulse within the decode window. In the write mode, the write precompensation circuit uses the data synchronizer to provide data encoding and independent late/early write precompensation for NRZ data. The ENDEC provides encoding and decoding, preferably of run length limited (RLL) signals. The control circuit coordinates and controls the operation of the aforementioned circuits and subsystems.
The trend towards higher storage media recording density and higher data rates has placed heavy demands on the signal detection process in a magnetic recording channel. Information is often provided to a read channel in a bit stream format. A bit stream consists of a series of logical ones or zeros presented in serial fashion. To accurately decode a serial bit stream, the read channel must be able to detect each individual bit. The size of the region on a magnetic recording disk used to represent a single bit has been greatly reduced. This has increased the density of information, and the storage capacity of disk drives. As the magnetic media recording density has increased signal processing techniques have been increasingly used to achieve the accuracy and reliability recording systems require. For example, in the past signal detection in magnetic disk drive read channels was based on peak detection. Peak detection has now been largely replaced by the partial response maximum likelihood (PRML) approach. For a discussion of recent PRML channel developments see G. J. Kerwin, et al., "Performance Evaluation of the Disk Industry's Second-Generation Partial Response Maximum Likelihood Data Channel," IEEE Trans. Magn., Vol. 27, No. 6, 4005 (November 1993). Variations in semiconductor processing techniques produce read channel circuits with varying circuit performance. To compensate for these variations the performance of each read channel is characterized and parameters are adjusted to optimize the channel.
The performance of a signal channel in, for example, a disk drive, is optimized by characterizing the effects of the channel on a signal and defining operating parameters based on that characterization. The optimization process is performed during factory calibration.
The expanded use of signal processing techniques requires that more parameters be determined in the optimization process. For example to optimize an advanced signal read channel may require determining the following signal processing parameters: continuous time cutoff frequency, frequency boost, transversal filter tap coefficients, and detector threshold values. In the past, one way magnetic disk drive read channels have been optimized is by incorporating a mean-squared error (MSE) in the channel electronics, and making bit error rate (BER) measurements of the disk drive. The MSE only provides an indirect measure of the bit error rate performance. This can cause the optimization process to produce less than optimum channel performance. The MSE measures the squared error between the received samples and the ideal target samples. Minimizing the MSE may not minimize the noise bandwidth. This would cause the signal-to-noise ratio to not be maximized, and a higher BER.
The BER is an ideal measure to use to optimize channel performance because it gives the actual performance of the channel. BER measurements are often made with pseudo-random signal patterns. When the BER is low, measuring it takes too much time to be practical. For example, to optimize a hard disk drive with a low BER may take up to five hours. One way to reduce optimization time is to increase the BER. The BER can be increased by adding noise to the test signal. In the production process, injecting noise from an external source is undesirable because it requires providing a connector to an external noise source or opening the drive under test. Providing a connector to an external noise source increases the disk drive manufacturing cost because of the increased production handling costs and additional production equipment. Injecting noise from an external source has the further drawback that the disk drive cannot readily be re-optimized in the field to compensate for component drift. Therefore, it would be desirable to provide a noise source to increase the BER without the drawbacks of the prior art.